1. Field of the Invention
The present invention relates to a lanthanum boride (LaB.sub.6) type single crystal and a method for growing the same.
2. Description of the Prior Art
A lanthanum boride single crystal is presently used as a thermionic electron emitter having a high brightness and a long life for a scanning electron microscope, an electron beam delineator and the like. For the electron emitting material, it is necessary to use a highly pure single crystal with high quality.
There are known solution method, gas phase method, fusion method and the like as a method for growing a lanthanum boride single crystal, but among them, fusion method using a high growing temperature is most suitable for growing a single crystal with high purity.
Examples of the fusion method include arc Verneuil's method, floating zone (FZ) method, pedestal method and the like, but among them, the floating zone method is most suitable for growing a signal crystal with high quality.
Heretofore, in the method for growing a lanthanum boride signal crystal by fusion method, a lanthanum boride starting material with high purity has been used for raising the purity of a single crystal. However, single crystals grown by this method often have a disadvantage of containing many defects (for example, a grain boundary density is 10.sup.3 cm/cm.sup.2). Accordingly, under the present circumstances, the crystal tips for the electron emitter must be cut out the high quality part so as not to contain the defects.
An object of the present invention is to provide a lanthanum boride single crystal with high quality having no defects by removing the disadvantages of the above mentioned conventional techniques.
In order to solve the above problems, we have studied the fact that many defects are present in a single crystal grown even by using a highly pure lanthanum boride starting material by the conventional fusion method, and have tried to incorporate other materials into lanthanum boride as a starting material. As this result, we have found that a grain boundary density in a single crystal is reduced when it is grown by using a starting material comprising lanthanum boride having a rare earth boride incorporated therein, the rare earth boride being selected from the group consisting of CeB.sub.6, PrB.sub.6, NdB.sub.6, SmB.sub.6, GdB.sub.6 and a mixture thereof. Particularly, we have found and achieved the present invention that a single crystal with high quality having no grain boundary can be obtained by incorporating the above mentioned rare earth boride in an amount of 20 mol % or higher into a lanthanum boride starting material when growing a single crystal having a diameter of 1 cm.